I. Technical Field
The present invention relates to a fixed type constant velocity universal joint which is used, for example, in the power transmitting system of automobiles and various industrial machines and which solely allows angular displacement between two shafts on the driving side and the driven side.
II. Background Art
A fixed type constant velocity universal joint is an example of a constant velocity universal joint used as means for transmitting torque from the engine of an automobile to the wheels at constant velocity. The fixed type constant velocity universal joint connects two shafts on the driving side and the driven side and is provided with a structure allowing constant-velocity transmission of rotational torque even when the two shafts assume an operating angle. Generally, as an example of the widely-known fixed type constant velocity universal joint described above, there may be given one adopting a birfield type (BJ) or an undercut free type (UJ).
FIGS. 16 and 17, and FIGS. 18 and 19 illustrate two examples of a constant velocity universal joint of a birfield type, for example. Those constant velocity universal joints each include an outer joint member 112 having an inner spherical surface 110 in which a plurality of track grooves 111 extending in an axial direction are formed at equal circumferential intervals, an inner joint member 115 having an outer spherical surface 113 in which a plurality of track grooves 114 extending in the axial direction while paired with the track grooves 111 of the outer joint member 112 are formed at equal circumferential intervals, a plurality of balls 116 interposed between the track grooves 111 of the outer joint member 112 and the track grooves 114 of the inner joint member 115, for transmitting a torque, and a cage 117 interposed between the inner spherical surface 110 of the outer joint member 112 and the outer spherical surface 113 of the inner joint member 115, for holding the balls 116.
The track grooves 111, 114 of the constant velocity universal joint have a single circular arc shape in an axial vertical section. A center curvature O1 of the track grooves 111 of the outer joint member 112 and a center curvature O2 of the track grooves 114 of the inner joint member 115 are offset from each other in a direction opposite to the axial direction by equal distances F, f with respect to a joint center O including a ball center O3 (track offset). Note that a center curvature of the inner spherical surface 110 of the outer joint member 112 (outer spherical surface 118 of cage 117) and a center curvature of the outer spherical surface 113 of the inner joint member 115 (inner spherical surface 119 of cage 117) correspond to the above-mentioned joint center O. As described above, owing to the provision of the track offset, a pair of the track grooves 111, 114 form a wedge-like ball track having radial intervals gradually becoming larger from the deep side of the outer joint member 112 toward the opening side.
When a constant velocity universal joint of this type is used, for example, for an automotive drive shaft, there is generally employed the structure in which the outer joint member 112 is connected to a driven shaft, and a drive shaft extending from a slide type constant velocity universal joint mounted to a differential on the vehicle body side is connected to the inner joint member 115 through spline fit-engagement. In this constant velocity universal joint, when an operating angle is assumed between the outer joint member 112 and the inner joint member 115, each of the balls 116 accommodated in the cage 117 is always maintained within the bisector plane of any operating angle, thereby securing the constant velocity property of the joint.
The plurality of balls 116 are arranged at equal circumferential intervals while accommodated in a pocket 120 formed in the cage 117. The constant velocity universal joint illustrated in FIGS. 16 and 17 has a structure in which six balls 116 are provided, and the constant velocity universal joint illustrated in FIGS. 18 and 19 has a structure in which eight balls 116 are provided. In the constant velocity universal joint of the eight ball type, the ball diameter is set smaller (d<D) and the track offset is set smaller (f<F) than those of the constant velocity universal joint of the six ball type. As a result, the compact constant velocity universal joint of high efficiency is realized.
FIG. 20 illustrates a state where an operating angle (40°, for example) is assumed in the constant velocity universal joint of the six ball type, and similarly, FIG. 21 illustrates a state where an operating angle (40°, for example) is assumed in the constant velocity universal joint of the eight ball type. As indicated with the broken line of FIG. 20, L6IN indicates a contact point trace in the angular contact between the inner joint member 115 and the ball 116, and L6OUT indicates a contact point trace in the angular contact between the outer joint member 112 and the ball 116. Further, as indicated with the broken line of FIG. 21, L8IN indicates a contact point trace in the angular contact between the inner joint member 115 and the ball 116, and L8OUT indicates a contact point trace in the angular contact between the outer joint member 112 and the ball 116.
In the constant velocity universal joint of the eight ball type illustrated in FIG. 21, as illustrated in FIG. 22, the ball diameter is set smaller. As a result, the length of the contact point trace L8IN of the inner joint member 115 and the ball 116, and the length of the contact point trace L8OUT of the outer joint member 112 and the ball 116 become smaller than those in the constant velocity universal joint of the six ball type (L8IN<L6IN, L8OUT<L6OUT). With this structure, the sliding speed between the track groove 111 of the outer joint member 112 and the ball 116 is reduced, whereby the torque transmission efficiency is increased.
Further, in the constant velocity universal joint of the eight ball type, the track offset is set smaller (f<F) as illustrated in FIG. 22. As a result, the length of the contact point trace L8IN of the inner joint member 115 and the ball 116, and the length of the contact point trace L8OUT of the outer joint member 112 and the ball 116 become smaller than those in the constant velocity universal joint of the six ball type (L8IN<L6IN, L8OUT<L6OUT). With this structure, the sliding speed between the track groove 111 of the outer joint member 112 and the ball 116 is reduced, whereby the torque transmission efficiency is increased.
Further, in the constant velocity universal joint of the eight ball type (refer to FIG. 18), the track offset is set smaller. As a result, a nip angle γ8 of the ball 116 with respect to each of the track grooves 111, 114 becomes smaller than that in the constant velocity universal joint of the six ball type (γ8<γ6) (refer to FIG. 16), and hence a force M8 for axially extruding the ball 116 to the outer joint member opening side is reduced (M8<M6).
Herein, the nip angles γ6 and γ8 of the ball 116 with respect to the track grooves 111, 114 represent angles each formed by two axial tangent lines at contact points (refer to broken lines of FIGS. 16 and 18) between the ball 116 and each of the track groove 111 of the outer joint member 112 and the track groove 114 of the inner joint member 115. Note that, in FIGS. 16 and 18, the broken line in the ball 116 indicates a contact point trace in the angular contact between the ball 116 and the track grooves 111, 114.
The force M8 for axially extruding the ball 116 to the outer joint member opening side is transmitted to the cage 117. As a result, in the constant velocity universal joint of the eight ball type, the spherical surface forces between the outer spherical surface 118 of the cage 117 and the inner spherical surface 110 of the outer joint member 112, and between the inner spherical surface 119 of the cage 117 and the outer spherical surface 113 of the inner joint member 115 become smaller than those in the constant velocity universal joint of the six ball type. With this structure, the frictional loss (heat generation) at the spherical surface contact portions are reduced, whereby the torque transmission efficiency is increased (refer to JP 3460107 B and JP 09-317784 A, for example).
Further, in order to secure the operability of the constant velocity universal joint, it is necessary to set a gap at each portion. For example, it is necessary to set a pitch circle diameter (PCD) gap and a spherical surface gap to appropriate values (refer to JP 2002-323061 A and 2005-188620 A, for example).
Herein, the PCD gap represents the difference between the PCD (outer joint member PCD) of the ball 116 in the state of being held in contact with the track groove 111 of the outer joint member 112 and the PCD (inner joint member PCD) of the ball 116 in the state of being held in contact with the track groove 114 of the inner joint member 115. Further, the spherical surface gap represents a gap between the outer spherical surface 113 of the inner joint member 115 and the inner spherical surface 119 of the cage 117, or a gap between the outer spherical surface 118 of the cage 117 and the inner spherical surface 110 of the outer joint member 112.
The PCD gap is set while taking into consideration the machining accuracy of the track grooves 114 of the inner joint member 115 and the track grooves 111 of the outer joint member 112, the operability, the damage caused by climbing up of the balls 116 from the track grooves 111, 114 at the used torque, the heat generation due to the frictional resistance in the track grooves 111, 114, the fatigue durability, and the like. Further, the spherical surface gap is similarly set while taking into consideration the machining accuracy of the outer spherical surface 113 of the inner joint member 115 and the inner spherical surface 110 of the outer joint member 112, the operability, the incorporation properties, and the like.